Route 378 bridge spans need different sizes of support

Q: I've noticed something unusual about the Route 378 overpass across Route 22. The girders at one end of the bridge are thicker (deeper?) than the girders at the other end. It seems that the three spans are equal in length, so why would thicker girders be required at one end than at the other?

Peter Gerber

Bethlehem

A: As something of a frustrated engineer, Peter, the Warrior is interested in this kind of quirky query.

It's kind of hard to see, actually. But you're right, with a little scrutiny, you can see a difference in the width -- engineers call it "depth," but in this case it's the height as viewed from Route 22 -- in the three sets of beams between the support pillars.

Viewing the bridge from the east, the thickest beams are on the right, or north end; those in the middle section are a little thinner, and on the southern end, they're the most slender -- by just a few inches, but it's noticeable with the naked eye.

The difference also is discernible by the "step" effect at the support pillars; you can see that the northern beams drop down somewhat, as an "L" or step joint is required to support the beams of different depths.

So why the need for different size beams? You touched on the answer in your question, Peter, when you said it seems like the three spans of the bridge are of equal length.

They're not, according to Kamlesh Ashar, assistant bridge engineer for PennDOT's District 5 based in Allentown.

Of the three spans, the northern section is the longest, at 67 feet, 7 inches, Ashar said. That span gets beams that are 39 inches deep to support the road.

"The longer the span, the bigger the beams," Ashar said, which makes sense even to the Warrior, who just couldn't get the hang of that silly slide rule.

There are other ways to deal with varying span lengths, Ashar said, including using beams of the same depth, but simply adding more of them for the longer spans.

The designers of the Route 378 bridge chose the larger beams in this instance, he said, probably because it was the most cost- effective way to go.

The Warrior steered into a problem, however, while checking other nearby bridges to compare the designs.

The Schoenersville Road bridge, just east of Route 378, appears to be very similar. To the naked eye it looks as though the southern span is a bit shorter than the center one, and the northern span slightly longer than the center, by about the same proportion as the 378 bridge.

The same type of concrete beams are used to support the road on Schoenersville, Peter. But here's the rub: They're all the same depth, and run straight across, with no "step" supports required.

So what's the difference?

While it's true that the spans are not equidistant, there's not as much variation in the relative lengths as there is on Route 378, said District 5 Bridge Engineer Karl W. Kroboth.

There are other differences as well, Kroboth said. The Schoenersville bridge is wider, and thus has more beams: 10 of them, as opposed to eight at 378. And, while they're all the same depth on Schoenersville, those beams are deeper than the ones on the 378 bridge, Kroboth said.

As you can see, it gets pretty complicated, Peter. In addition to the width of the bridge being a variable, the angle at which the bridge crosses over the road also comes into play, as do the required loads, height allowances and other factors.

And incidentally, those girders don't rest directly on the concrete pillars. They sit on neoprene pads that allow the components to flex in reaction to changing temperatures, shifting vehicle loads and other variables.

Whew. Come to think of it, maybe the Warrior isn't so disappointed at missing out on a career as an engineer or architect.

Comfort's hard to find where there's so little room for error.

Road Warrior appears Fridays. E-mail questions about transportation in the Lehigh Valley and beyond to hartzell@mcall.com (please include your name, phone number and where you live). Or, write to Road Warrior, The Morning Call, 101 N. Sixth St., Allentown, 18101-1480.